There is a need to economically collect solar energy in concentrated form prior to direct use or conversion to electricity or other useable forms of energy. Solar energy has a modest intensity at the earth's surface of about 1000 Watts per square meter.
It is thus highly desirable to concentrate the energy to higher intensity (usually expressed in Watts per square meter, or W/m2) before use. This is particularly so where solar cells are used to convert the solar energy into electrical energy. The photovoltaic cells that are used to convert the solar energy into electrical energy are relatively expensive. Concentration of the incident solar energy into a smaller area allows the use of a smaller area of energy conversion cells, with lower resulting costs of conversion cells. The key requirement in a concentrating collector is a means to concentrate the energy as much as possible with a system which is very low in cost per unit area and which can track the sun by rotation about one or two axes.
In the past, various forms of concentrator have been used. These have included refractive concentrators (lenses) and, more commonly, curved reflectors (mirrors). The concentrators are generally mounted on structures that allow movement to follow or track the movement of the sun accurately across the sky each day. For economy the system used to track the sun must be as simple and robust as possible. Present methods of tracking use either motors and gears, or sliding hydraulic actuators, both of which add considerable cost. The need for tracking makes the concentrator structures heavier and more complex than static non-concentrating solar energy collectors, because the tracking movements usually require that all support be provided through rotary joints which are subject to very high forces during extreme winds.
Any structures used for concentration must be well protected from high winds, hail and other aspects of extreme weather conditions. It is also most commonly desirable to provide some form of cooling of the devices that convert the concentrated solar energy to electricity.
Silicon photovoltaic cells, which are the most economical variety at this point in time, operate less efficiently as their temperature increases. If a mechanism for cooling the cells is not used, the use of concentrators tends to cause the cells to operate at higher temperature, decreasing their energy conversion efficiency.
Most concentrators designed so far employ very substantial mechanical structures to resist movement and damage from the wind. In addition they usually employ heavy and strong materials such as glass with metal backing for the reflective element to protect the device from damage by wind, ice and hail. Such structures are, at present, either very expensive or too fragile for continuous outdoor use.
One method that has been used to protect the reflective concentrator surface has been the use of an inflatable, aluminised, flexible plastic membrane as a concentrating reflector. The shape of the membrane is maintained by an air pressure difference from one side to the other. Such reflectors can be deflated during severe weather. They are relatively cheap, but are still subject to damage from high winds and by ultraviolet light. In addition they require substantial structures to support the moving parts against high winds.
Another method that has been used to protect concentrators is the use of a transparent dome or building to cover and protect the whole solar concentrator. This does allow some simplification of the structural design of the moving concentrator. However, this method has little or no overall cost advantage due to the added cost of the protective structure.
The Yeomans patents WO93/09390 and U.S. Pat. No. 6,220,241 B1 use temporary immersion in water to protect a reflective concentrator. It consists of a reflective concentrator floating on water with a heat collector at the focal point in the air above. The concentrator can be submerged using pumps for limited periods to avoid damage to the concentrator mirror during adverse weather conditions. This is achieved by flooding its' buoyancy tanks with water, a change in the absolute buoyancy of the apparatus. It is not able to operate as a solar concentrator or energy collector while submerged. The concentrator may still be damaged in bad weather if the mechanism fails or loses power at a critical time (it is not passively robust). This system also achieves tracking movement in an azimuth direction (rotation about the vertical axis only), using movement within the water. Tracking about a horizontal axis is achieved via motors, gears and levers.
Virtually all the existing concentrating collectors require the mechanism to move to a special protected position for protection against adverse weather, making them particularly vulnerable to damage when there is a mechanical or electrical breakdown.
Russian patent number SU1430-927-A to Novorossiisk Naval describes the general concept of floating a flexible transparent sack in water to create a lens, but no details are given of the material to fill this sack, or of any energy collector or conversion device, or of the scale of the device and no tracking method is proposed.
Aims
The present invention accordingly aims to provide protection against weather conditions and ultraviolet radiation for solar concentrators and collectors. Subsidiary aims of the present invention are, to provide a simple means for tracking solar concentrators to follow the sun and to provide cooling for solar collectors, and to provide more lightweight structures than was previously possible. The invention addresses these aims, at least in part, by using the protective, cooling and buoyancy properties of a body of liquid, such as a lagoon, pond, tank, lake, dam or the like of water or other liquid.